System and method for controlling valve timing of continuous variable valve duration engine

ABSTRACT

The present disclosure provides a system and a method for controlling valve timing of a continuous variable valve duration engine. The method includes: classifying control regions based on engine speed and load; retarding intake valve closing (IVC) timing, applying a long duration to an exhaust valve, and limiting a valve overlap in a first control region; advancing the IVC timing, applying the long duration to the exhaust valve, and controlling the valve overlap in a second control region; applying the long duration to the exhaust valve and advancing the IVC timing in a third control region; controlling a throttle valve to be fully opened and applying a short duration to the exhaust valve in a fourth control region; and controlling the throttle valve to be fully opened, applying the long duration to the exhaust valve, and retarding the IVC timing in a fifth control region.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2016-0031659, on Mar. 16, 2016, the entire contentsof which are incorporated herein by reference.

FIELD

The present disclosure relates to a system and a method for controllingvalve timing of a continuous variable valve duration engine.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

An internal combustion engine combusts mixed gas in which fuel and airare mixed at a predetermined ratio through a set ignition mode togenerate power by using explosion pressure.

Generally, a camshaft is driven by a timing belt connected with acrankshaft that converts linear motion of a piston by the explosionpressure into rotating motion to actuate an intake valve and an exhaustvalve, and while the intake valve is opened, air is suctioned into acombustion chamber, and while an exhaust valve is opened, gas which iscombusted in the combustion chamber is exhausted.

To improve the operations of the intake valve and the exhaust valve andthereby improve engine performance, a valve lift and a valveopening/closing time (timing) are controlled according to a rotationalspeed or load of an engine. Therefore, a continuous variable valveduration (CWD) device controlling opening duration of an intake valveand an exhaust valve of the engine and a continuous variable valvetiming (CWT) device controlling opening timing and closing timing of theintake valve and the exhaust valve of the engine have been developed.

The CWD device adjusts opening duration (opening time) of the valve. Inaddition, the CVVT device advances or retards opening and closing timingof the valve in a state in which the duration of the valve is fixed. Inother words, when the opening timing of the valve is determined, theclosing timing is automatically determined according to the duration ofthe valve.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the disclosure andtherefore it may contain information that does not form the prior artthat is already known to a person of ordinary skill in the art.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

SUMMARY

The present disclosure provides a system and method for controllingvalve timing of a continuous variable valve duration engine havingadvantages of simultaneously controlling duration and timing of thecontinuous variable valve by mounting a continuous variable valve timingdevice on an intake and mounting a two-stage variable valve durationdevice and a continuous variable valve timing device on an exhaust.

A method for controlling valve timing of an engine provided with acontinuous variable valve timing (CWT) device at an intake and atwo-stage variable valve duration (WD) device and a continuous variablevalve timing (CWT) device at an exhaust according to one form of thepresent disclosure may include: classifying a plurality of controlregions depending on an engine speed and an engine load; retarding anintake valve closing (IVC) timing, applying a long duration to anexhaust valve, and limiting a valve overlap between an intake valve andthe exhaust valve in a first control region; advancing the IVC timing,applying the long duration to the exhaust valve, and controlling thevalve overlap according to the engine load in a second control region;applying the long duration to the exhaust valve and advancing the IVCtiming according to an increase of the engine load in a third controlregion; controlling a throttle valve to be fully opened and applying ashort duration to the exhaust valve in a fourth control region; andcontrolling the throttle valve to be fully opened, applying the longduration to the exhaust valve, and retarding the IVC timing according toan increase of the engine speed in a fifth control region.

Exhaust valve closing (EVC) timing may be set as a maximum value capableof maintaining combustion stability in the first control region.

The valve overlap may be increased by retarding the exhaust valveclosing (EVC) timing until the engine load reaches a predetermined loadand the valve overlap may be reduced by advancing the EVC timing whenthe engine load is increased over the predetermined load in the secondcontrol region.

Exhaust valve opening (EVO) timing may be controlled to be close tobottom dead center in the fourth control region.

A system for controlling valve timing of a continuous variable valveduration engine according to one form of the present disclosure mayinclude: a data detector detecting data related to a running state of avehicle; a camshaft position sensor detecting a position of a camshaft;an intake continuous variable valve timing (CVVT) device controllingopening timing and closing timing of an intake valve of the engine; anexhaust two-stage variable valve duration (VVD) device controllingopening duration of an exhaust valve of the engine in two stages; anexhaust continuous variable valve timing (CWT) device controllingopening timing and closing timing of the exhaust valve; and a controllerconfigured to classify a plurality of control regions depending on anengine speed and an engine load based on signals from the data detectorand the camshaft position sensor, and configured to control operationsof the intake CWT device, the exhaust two-stage WD device and theexhaust CVVD device according to the control regions.

The controller retards the intake valve closing (IVC) timing, applies along duration to the exhaust valve and limits a valve overlap betweenthe intake valve and the exhaust valve in a first control region,advances the IVC timing, applies the long duration to the exhaust valveand controls the valve overlap according to the engine load in a secondcontrol region, applies the long duration to the exhaust valve andadvances the IVC timing according to an increase of the engine load in athird control region, controls a throttle valve to be fully opened andapplies a short duration to the exhaust valve in a fourth controlregion, and controls the throttle valve to be fully opened, applies thelong duration to the exhaust valve and retards the IVC timing accordingto an increase of the engine speed in a fifth control region.

The controller may set the exhaust valve closing (EVC) timing as amaximum value capable of maintaining combustion stability in the firstcontrol region.

The controller may increase the valve overlap by retarding exhaust valveclosing (EVC) timing until the engine load reaches a predetermined loadand may reduce the valve overlap by advancing the EVC timing when theengine load is increased over the predetermined load in the secondcontrol region.

The controller may control the exhaust valve opening (EVO) timing to beclose to bottom dead center in the fourth control region.

According to one form of the present disclosure, duration and timing ofthe continuous variable valve are simultaneously controlled, so theengine may be controlled under desirable conditions.

That is, since the opening timing and closing timing of the intake valveand the exhaust valve are appropriately controlled, the fuel efficiencyunder a partial load condition and power performance under a high loadcondition are improved. In addition, a fuel amount for starting may bereduced by increasing a valid compression ratio, and exhaust gas may bereduced by shortening time for heating a catalyst.

Further, since the two-stage variable valve duration device is usedinstead of a continuous variable valve duration device at the exhaust,production cost reduced while maintaining power performance.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

In order that the disclosure may be well understood, there will now bedescribed various forms thereof, given by way of example, referencebeing made to the accompanying drawings, in which:

FIG. 1 is a schematic block diagram showing a system for controllingvalve timing of a continuous variable valve duration engine according toone form of the present disclosure;

FIG. 2 is a perspective view showing an intake provided with acontinuous variable valve timing device and an exhaust provided with atwo-stage variable valve duration device and a continuous variable valvetiming device according to one form of the present disclosure;

FIG. 3A and FIG. 3B are flowcharts showing a method for controllingvalve timing of a continuous variable valve duration engine according toone form of the present disclosure;

FIGS. 4A-4C are graphs showing duration, opening timing, and closingtiming of an intake valve depending on an engine load and an enginespeed according to the present disclosure; and

FIGS. 5A-5C are graphs showing duration, opening timing, and closingtiming of an exhaust valve depending on an engine load and an enginespeed according to the present disclosure.

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

As those skilled in the art would realize, the described forms may bemodified in various different ways, all without departing from thespirit or scope of the present disclosure.

Throughout this specification and the claims which follow, unlessexplicitly described to the contrary, the word “comprise” and variationssuch as “comprises” or “comprising” will be understood to imply theinclusion of stated elements but not the exclusion of any otherelements.

It is understood that the terms “vehicle” or “vehicular” or othersimilar terms as used herein are inclusive of motor vehicles in generalincluding hybrid vehicles, plug-in hybrid electric vehicles, and otheralternative fuel vehicles (e.g., fuels derived from resources other thanpetroleum). As referred to herein, a hybrid electric vehicle is avehicle that has two or more sources of power, for example agasoline-powered and electric-powered vehicle.

Additionally, it is understood that some of the methods may be executedby at least one controller. The term controller refers to a hardwaredevice that includes a memory and a processor configured to execute oneor more steps that should be interpreted as its algorithmic structure.The memory is configured to store algorithmic steps, and the processoris specifically configured to execute said algorithmic steps to performone or more processes which are described further below.

Furthermore, the control logic of the present disclosure may be embodiedas non-transitory computer readable media on a computer readable mediumcontaining executable program instructions executed by a processor, acontroller, or the like. Examples of computer readable media include,but are not limited to, ROM, RAM, compact disc (CD)-ROMs, magnetictapes, floppy disks, flash drives, smart cards, and optical data storagedevices. The computer readable recording medium can also be distributedin network coupled computer systems so that the computer readable mediais stored and executed in a distributed fashion, e.g., by a telematicsserver or a controller area network (CAN).

FIG. 1 is a schematic block diagram showing a system for controllingvalve timing of a continuous variable valve duration engine according toan exemplary form of the present disclosure.

As shown in FIG. 1, a system for controlling valve timing of acontinuous variable valve duration engine includes a data detector 10, acamshaft position sensor 20, a controller 30, an intake continuousvariable valve timing device 45, an exhaust two-stage variable valveduration device 50, an exhaust continuous variable valve timing device55, and a throttle valve 60, although other sensors or systems may beemployed to detect or determine the desired data.

The data detector 10 detects data related to a running state of thevehicle for controlling the CWD device and the CVVT devices, andincludes a vehicle speed sensor 11, an engine speed sensor 12, an oiltemperature sensor 13, an air flow sensor 14, and an accelerator pedalposition sensor (APS) 15.

The vehicle speed sensor 11 detects a vehicle speed, and transmits asignal corresponding thereto to the controller 30. The vehicle speedsensor 11 may be mounted at a wheel of the vehicle.

The engine speed sensor 12 detects an engine speed from a change inphase of a crankshaft or camshaft, and transmits a signal correspondingthereto to the controller 30.

The oil temperature sensor (OTS) 13 detects temperature of oil flowingthrough an oil control valve (OCV), and transmits a signal correspondingthereto to the controller 30.

The oil temperature detected by the oil temperature sensor 13 may bedetermined by determining a coolant temperature using a coolanttemperature sensor mounted at a coolant passage of an intake manifold.Therefore, in one form, the oil temperature sensor 13 may include thecoolant temperature sensor, and the oil temperature should be understoodto be the coolant temperature.

The air flow sensor 14 detects an air amount flowing into the intakemanifold, and transmits a signal corresponding thereto to the controller30.

The accelerator pedal position sensor 15 detects a degree at which adriver pushes an accelerator pedal, and transmits a signal correspondingthereto to the controller 30. The position value of the acceleratorpedal is 100% when the accelerator pedal is pressed fully, and theposition value of the accelerator pedal is 0% when the accelerator pedalis not pressed at all.

The throttle valve position sensor that is mounted on an intake passagemay be used instead of the accelerator pedal position sensor 15.Therefore, in one form, the accelerator pedal position sensor 15 mayinclude the throttle valve position sensor, and the position value ofthe accelerator pedal should be understood to be an opening value of thethrottle value.

The camshaft position sensor 20 detects a position of a camshaft angle,and transmits a signal corresponding thereto to the controller 30.

FIG. 2 is a perspective view showing an intake provided with acontinuous variable valve timing device and an exhaust provided with atwo-stage variable valve duration device and a continuous variable valvetiming device according to one form of the present disclosure.

As shown in FIG. 2, the continuous variable valve timing device ismounted on the intake, and the two-stage variable valve duration deviceand the continuous variable valve timing device are mounted on theexhaust. Therefore, the intake valve duration (IVD) is fixed. If the IVDbecomes long, fuel efficiency and high speed performance of the vehiclemay be improved, but low speed performance may be deteriorated. Thus,the IVD may be fixed at an angle of approximately 250 to 260 degrees.

The intake continuous variable valve timing (CWT) device 45 controls anopening timing and a closing timing of an intake valve of the engineaccording to a signal from the controller 30, and the exhaust continuousvariable valve timing (CVVT) device 55 controls an opening timing and aclosing timing of an exhaust valve of the engine according to a signalfrom the controller 30.

The exhaust two-stage variable valve duration (WD) device 50 controls anopening duration of the exhaust valve of the engine in two stagesaccording to a signal from the controller 30. Since a two-stage WDdevice 50 operated by a solenoid valve is used instead of a CWD device,a motor and a sensor for operating the CWD device can be omitted,thereby reducing production cost.

If exhaust valve duration (EVD) is becomes long, fuel efficiency andhigh speed performance of the vehicle may be improved, but low speedperformance may be deteriorated. Thus, a short duration for low speedperformance and a long duration for high speed performance may be set.For example, the short duration may be set at an angle of approximately180 to 210 degrees, and the long duration may be set at an angle ofapproximately 240 to 250 degrees.

The exhaust two-stage VVD device 50 may apply the short duration and thelong duration to the exhaust valve by switching.

The throttle valve 60 adjusts air amount flowing into the intakemanifold.

The controller 30 classifies a plurality of control regions depending onan engine speed and an engine load based on signals of the data detector10 and the camshaft position sensor 20, and controls operations of theintake CWT device 45, the exhaust two stage VVD device 50, the exhaustCWT device, and the throttle valve 60 according to the control regions.Herein, the plurality of control regions may be classified into fiveregions.

The controller 30 retards an intake valve closing (IVC) timing, appliesthe long duration to the exhaust valve and limits a valve overlap in afirst control region, advances the IVC timing, applies the long durationto the exhaust valve and controls the valve overlap according to theengine load in a second control region, applies the long duration to theexhaust valve and advances the IVC timing according to an increase ofthe engine load in a third control region, controls the throttle valveto be fully opened and applies the short duration to the exhaust valvein a fourth control region, and controls the throttle valve to be fullyopened, applies the long duration to the exhaust valve and retards theIVC timing according to an increase of the engine speed in a fifthcontrol region.

For these purposes, the controller 30 may be implemented with at leastone processor executed by a predetermined program, and the predeterminedprogram may programmed in order to perform each step of a method forcontrolling valve timing of a continuous variable valve duration engine.

Various forms described herein may be implemented within a recordingmedium that may be read by a computer or a similar device by usingsoftware, hardware, or a combination thereof.

For example, the hardware of the forms described herein may beimplemented by using at least one of application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), fieldprogrammable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, and electrical units designed toperform any other functions.

The software such as procedures and functions described in the presentdisclosure may be implemented by separate software modules. Each of thesoftware modules may perform one or more functions and operationsdescribed in the present disclosure. A software code may be implementedby a software application written in an appropriate program language.

Hereinafter, a method for controlling valve timing of a continuousvariable valve duration engine according to one form of the presentdisclosure will be described in detail with reference to FIG. 3A to FIG.5C.

FIGS. 3A and 3B are flowcharts showing a method for controlling valvetiming of a continuous variable valve duration engine. In addition,FIGS. 4A-4C are graphs showing duration, opening timing, and closingtiming of an intake valve depending on an engine load and an enginespeed, and FIGS. 5A-5C are graphs showing duration, opening timing, andclosing timing of an exhaust valve depending on an engine load and anengine speed.

As shown in FIGS. 3A and 3B, a method for controlling valve timing of acontinuous variable valve duration engine begins with classifying aplurality of control regions depending on an engine load and an enginespeed at step S100. The first to fifth control regions are indicated inFIG. 4A to FIG. 5C.

The controller 30 may classify control regions as the first controlregion when the engine load is less than a first predetermined load, thesecond control region when the engine load is equal to or greater thanthe first predetermined load and less than a second predetermined load,and the third control region when the engine load is equal to or greaterthan the second predetermined load and less than a third predeterminedload. In addition, the controller 30 may classify control regions as thefourth control region when the engine load is equal to or greater thanthe second predetermined load and the engine speed is less than apredetermined speed, and the fifth control region when the engine loadis equal to or greater than the third predetermined load and the enginespeed is equal to or greater than the predetermined speed.

Meanwhile, as shown in FIG. 4A to FIG. 5C, a crank angle is indicated inan intake valve duration (IVD) map and an exhaust valve duration (EVD)map. In addition, a number designated in an intake valve opening (IVO)timing map represents before top dead center (TDC), a number designatedan intake valve closing (IVC) timing map represents after bottom deadcenter (BDC), a number designated in an exhaust valve opening (EVO)timing map represents before BDC, and a number designated in an exhaustvalve closing (EVC) timing map represents after TDC. Regions and curvedlines shown in FIG. 4A to FIG. 5C are just examples for describing oneform of the present disclosure, and the present disclosure is notlimited thereto.

When the control regions are classified depending on the engine load andthe engine speed at step S100, the controller 30 determines whether acurrent engine state belongs to the first control region at step S110.

When the engine load is less than the first predetermined load at stepS110, the controller 30 determines that the current engine state belongsto the first control region. In this case, the controller 30 retards theIVC timing, applies the long duration to the exhaust valve, and controlsthe valve overlap between the intake valve and the exhaust valve at stepS120. The valve overlap represents a state in which the intake valve isopened and the exhaust valve is not yet closed.

When the engine is operated at a low load condition, fuel efficiency maybe improved by maximally retarding the IVC timing. Accordingly, thecontroller 30 retards the IVC timing to an angle of approximately 100degrees after bottom dead center (i.e., a late intake valve close (LIVC)position).

In addition, the controller 30 may apply the long duration to theexhaust valve and may set the EVC timing as a maximum value capable ofmaintaining combustion stability such that the valve overlap is limited.In this case, since the long duration is applied to the exhaust valve,the controller 30 controls an exhaust valve opening (EVO) timing to anangle of approximately 50 degrees before bottom dead center (BDC).

When the current engine state does not belong to the first controlregion at step S110, the controller 30 determines whether the currentengine state belongs to the second control region at step S130.

When the engine load is equal to or greater than the first predeterminedload and less than the second predetermined load at step S130, thecontroller 30 determines that the current engine state belongs to thesecond control region. In this case, the controller 30 advances the IVCtiming, applies the long duration to the exhaust valve, and controls thevalve overlap according to the engine load at step S140.

When the IVC timing is the LIVC position in the first control region,the valve overlap is limited. The controller 30 may generate the valveoverlap by advancing the IVC timing in the second control region.

The controller 30 may increase the valve overlap by retarding the EVCtiming in an after TDC direction until the engine load reaches apredetermined load.

When the EVC timing is retarded in the after TDC direction, as the valveoverlap is increased, intake pumping may be decreased, however, sincethe EVO timing is close to BDC according to the long duration, exhaustpumping may be increased. Accordingly, when the engine load is increasedover the predetermined load, the controller 30 may reduce the valveoverlap by advancing the EVC timing toward a locking position.

When the current engine state does not belong to the second controlregion at step S130, the controller 30 determines whether the currentengine state belongs to the third control region at step S150.

When the engine load is equal to or greater than the secondpredetermined load and less than the third predetermined load, thecontroller 30 determines that the current engine state belongs to thethird control region. In this case, the controller 30 applies the longduration to the exhaust valve and advances the IVC timing according tothe increase of the engine load at step S160.

Since the valve overlap is increased as the IVC timing is advanced, thecontroller 30 may reduce the valve overlap by fixing the EVC timing atthe locking position.

The long duration is applied to the exhaust valve in the second andthird control regions. The exhaust pumping needs to be decreased and thevalve overlap needs to be increased in the second control region, butthe valve overlap needs to be decreased in the third control region.

When the current engine state does not belong to the third controlregion at step S150, the controller 30 determines whether the currentengine state belongs to the fourth control region at step S170.

The controller 30 determines that the current engine state belongs tothe fourth control region when the engine load is equal to or greaterthan the second predetermined load and the engine speed is less than thepredetermined speed. In this case, the controller 30 controls thethrottle valve to be fully opened and applies the short duration to theexhaust valve at step S180.

Since the engine speed is less than the predetermined speed (e.g.,approximately 1500 rpm) in the fourth control region, a scavengingphenomenon in which combustion gas is emitted due to reduction ofexhaust port pressure is generated by using exhaust interferencereduction.

The EVC timing is controlled after TDC to generate the scavengingphenomenon, and effect of the scavenging phenomenon may be maximizedwhen exhaust interference is reduced by controlling the EVO timing to beclose to BDC. For this purpose, the controller 30 may apply the shortduration to the exhaust valve instead of the long duration. Withreference to switching line shown in FIGS. 5A-5C, a left side of theswitching line is a region to which the short duration is applied, and aright side of the switching line is a region to which the long durationis applied.

When the current engine state does not belong to the fourth controlregion at step S170, the controller 30 determines whether the currentengine state belongs to the fifth control region at step S190.

When the engine load is equal to or greater than the third predeterminedload and the engine speed is equal to or greater than the predeterminedspeed at step S190, the controller 30 determines that the current enginestate belongs to the fifth control region. In this case, the controller30 controls the throttle valve to be fully opened, applies the longduration to the exhaust valve, and retards the IVC timing according tothe increase of the engine speed at step S200.

Since the engine speed is equal to or greater than the predeterminedspeed (e.g., approximately 1500 rpm) in the fifth control region, thescavenging phenomenon disappears or is reduced, and thus the longduration of the exhaust valve increases exhaust pumping. The controller30 may apply the long duration to the exhaust valve by switching theshort duration applied in the fourth control region, and retards the IVCtiming according to the increase of the engine speed.

The IVC timing may be gradually retarded from an angle of approximately20 degrees to an angle of approximately 60 degrees after BDC accordingto the increase of the engine speed.

As described above, according to one form of the present disclosure,duration and timing of the continuous variable valve are simultaneouslycontrolled, so the engine may be controlled under desirable conditions.

That is, since the opening timing and closing timing of the intake valveand the exhaust valve are appropriately controlled, the fuel efficiencyunder a partial load condition and power performance under a high loadcondition are improved. In addition, a fuel amount for starting may bereduced by increasing a valid compression ratio, and exhaust gas may bereduced by shortening time for heating a catalyst.

Further, since the two-stage variable valve duration device is usedinstead of a continuous variable valve duration device at the exhaust,production cost reduced while maintaining power performance.

While this disclosure has been described in connection with what ispresently considered to be practical forms, it is to be understood thatthe disclosure is not limited to the disclosed forms, but, on thecontrary, is intended to cover various modifications and equivalentarrangements included within the spirit and scope of the presentdisclosure.

The description of the disclosure is merely exemplary in nature and,thus, variations that do not depart from the substance of the disclosureare intended to be within the scope of the disclosure. Such variationsare not to be regarded as a departure from the spirit and scope of thedisclosure.

What is claimed is:
 1. A method for controlling valve timing of an engine provided with a continuous variable valve timing (CWT) device at an intake and a two-stage variable valve duration (WD) device and a continuous variable valve timing (CVVT) device at an exhaust, the method comprising: classifying, by a controller, a plurality of control regions depending on an engine speed and an engine load; retarding, by the controller, an intake valve closing (IVC) timing, applying a long duration to an exhaust valve, and limiting a valve overlap between an intake valve and the exhaust valve in a first control region; advancing, by the controller, the IVC timing, applying the long duration to the exhaust valve, and controlling the valve overlap according to the engine load in a second control region; applying, by the controller, the long duration to the exhaust valve and advancing the IVC timing according to an increase of the engine load in a third control region; controlling, by the controller, a throttle valve to be fully opened and applying a short duration to the exhaust valve in a fourth control region; and controlling, by the controller, the throttle valve to be fully opened, applying the long duration to the exhaust valve, and retarding the IVC timing according to an increase of the engine speed.
 2. The method of claim 1, wherein an exhaust valve closing (EVC) timing is set as a maximum value capable of maintaining combustion stability in the first control region.
 3. The method of claim 1, wherein the valve overlap is increased by retarding an exhaust valve closing (EVC) timing until the engine load reaches a predetermined load and the valve overlap is reduced by advancing the EVC timing when the engine load is increased over the predetermined load in the second control region.
 4. The method of claim 1, wherein an exhaust valve opening (EVO) timing is controlled to be close to a bottom dead center in the fourth control region.
 5. A system for controlling valve timing of a continuous variable valve duration engine, the system comprising: a data detector configured to detect data related a running state of a vehicle; a camshaft position sensor configured to detect a position of a camshaft; an intake continuous variable valve timing (CWT) device configured to control an opening timing and a closing timing of an intake valve of the engine; an exhaust two-stage variable valve duration (VVD) device configured to control an opening duration of an exhaust valve of the engine in two stages; an exhaust continuous variable valve timing (CWT) device configured to control an opening timing and a closing timing of the exhaust valve; and a controller configured to classify a plurality of control regions depending on an engine speed and an engine load based on signals from the data detector and the camshaft position sensor, and configured to control operations of the intake CVVT device, the exhaust two-stage WD device and the exhaust CWD device according to the control regions; wherein the controller retards an intake valve closing (IVC) timing, applies a long duration to the exhaust valve and limits a valve overlap between the intake valve and the exhaust valve in a first control region, advances the IVC timing, applies the long duration to the exhaust valve and controls the valve overlap according to the engine load in a second control region, applies the long duration to the exhaust valve and advances the IVC timing according to an increase of the engine load in a third control region, controls a throttle valve to be fully opened and applies a short duration to the exhaust valve in a fourth control region, and controls the throttle valve to be fully opened, applies the long duration to the exhaust valve and retards the IVC timing according to an increase of the engine speed in a fifth control region.
 6. The system of claim 5, wherein the controller sets an exhaust valve closing (EVC) timing as a maximum value capable of maintaining combustion stability in the first control region.
 7. The system of claim 5, wherein the controller increases the valve overlap by retarding an exhaust valve closing (EVC) timing until the engine load reaches a predetermined load and reduces the valve overlap by advancing the EVC timing when the engine load is increased over the predetermined load in the second control region.
 8. The system of claim 5, wherein the controller controls an exhaust valve opening (EVO) timing to be close to a bottom dead center in the fourth control region. 